Porges’s work—which noted researcher Paul Ekman called “a truly revolutionary perspective on human nature”—dramatically broadens our understanding of the sympathetic and parasympathetic systems, and explains how our bodies and brains interact with one another to regulate our physiological states. However, what may be more pertinent to therapists is the extent to which our autonomic nervous systems influence long-term issues with intimacy and trust. In the interview that follows, Porges offers some research-based insights into how therapists can more effectively convey safety to clients and clarifies the evolutionary roots of anxiety, depression, and trauma.

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RH: Can you explain your Polyvagal Theory in simple terms?

Porges: It’s hard to make it simple, but let’s try by starting with what we’ve all learned about the autonomic nervous system. It’s a pair of antagonistic systems: the sympathetic supports mobilization, and the parasympathetic supports immobilization, usually associated with relaxation, growth, and restoration. In the past, we tended to believe that stress responses were, in general, vested within the sympathetic nervous system’s capacity to support fight-or-flight behaviors. But there’s another defense system, unrelated to the sympathetic nervous system and dependent on the parasympathetic nervous system. The mechanisms and adaptive function of this defense system are impossible to understand from the paired antagonism model. The parasympathetic defense response is mediated through a vagal circuit producing a behavioral shutdown such as fainting or, from a clinical perspective, dissociation. This defense system doesn’t fit within the fight-or-flight model. Nor does it fit within the view that the vagus, the major nerve in the parasympathetic nervous system, mediates calmness and induces resilience and health.

RH: Your work suggests that our autonomic systems are better thought of as hierarchical, rather than competing.

Porges: Right. The vertebrate autonomic nervous system has changed through stages of evolution, and the human autonomic nervous system shares several of these autonomic circuits with more ancient vertebrates. Functionally, our autonomic nervous system is composed of three phylogenetically organized subsystems. We utilize our newest systems first, and when they don’t work, we recruit older ones. In terms of evolution, the newest autonomic circuit is a uniquely mammalian vagal circuit, which inhibits the heart rate by placing a tonic inhibition on the heart’s pacemaker. This circuit also inhibits sympathetic activity. The brainstem areas controlling this neural pathway coordinate the nerves controlling the muscles in the face and head. So people are literally showing their heart on their face. That’s because humans are social beings who have to convey to one another that we’re safe to come close to, to hug, and in some cases, to have sex with. To convey this message of safety, we utilize the newest vagal circuit to down-regulate our sympathetic defenses and present cues of safety when it’s appropriate. While the face is a crucial vehicle for this, the voice also plays an important role in conveying a physiological state of calm. If the voice has a higher-pitched frequency, it’s saying, “Don’t come near me.”

The thing to bear in mind is that the vagal circuit is both expressive and receptive. That’s why you feel calmer when I use a soothing, prosodic voice. When the vagal circuit is working, our middle-ear muscles change our capacity to hear predators or low-frequency sounds. Middle-ear muscles, similar to the muscles of the face, are regulated by the brainstem area that controls the mammalian vagal circuit. Typically, when there’s something in the environment that threatens us, we turn off the vagal circuit, because it inhibits our ability to mobilize: it gets in the way of moving to fight or to flee.